1. Field of the Invention
This invention relates to an optical sensor for monitoring in-flight particles in thermal spray processes and other industrial processes.
Thermal spraying in general, and plasma spraying in particular, is a powerful technique widely used to produce protective coatings on a large variety of substrates. For example, thermal barrier coatings are plasma sprayed in producing aircraft engines and ceramic and metal coatings are thermally sprayed for various purposes.
The properties of coatings depend upon many spraying parameters, some of them being related to the spray gun operation. Consequently, spraying process control has been implemented by monitoring and regulating gun input variables. In plasma spraying, parameters such as arc current and power, arc gas flow rates, powder feed rate, and powder carrier gas pressure are controlled to keep them at predetermined optimum values. This control approach has been found to be complex because a large number of interrelated input variables must be monitored, and has been found to be incomplete because some variables, such as electrode wear state, cannot be monitored at all.
An alternative control approach is described in U.S. Pat. No. 5,180,921, in which the temperature and velocity of the sprayed particles are monitored before their impingement on the substrate. On-line measurement of these particle parameters, which directly influence the structure of the sprayed coatings, can provide an efficient feedback signal generator to perform feedback for the gun input parameters and a diagnostic tool to detect any problem during the coating operation.
Collecting information about particle flow is also useful in other industrial applications. For example, production of metallic powders by gas atomization involves the atomization of a molten metal by a series of gas jets and on-line measurement of the particle temperature, velocity and diameter provides key information about the state of the process.
2. Description of the Prior Art
Different techniques exist for measuring the diameter of in-flight particles in industrial environments. Some techniques are based on laser beam illumination of the in-flight particles to obtain particle characteristics. For example, dual beam laser Doppler anemometry has been proposed by M. J. Rudd (U.S. Pat. No. 3,680,961) and by R. Adrian and K. L. Orion in Applied Optics, 16 (1977) 677-684 to simultaneously measure the size and velocity of moving particles. D. J. Holve and K. D. Annen in Optical Engineering, 23 (1984) 591-603 described a different arrangement in which a laser beam is used to illuminate the moving particles and the scattered radiation is detected in the forward or backward direction. The particle size and velocity are obtained after deconvolution of the detected signals. To simplify the treatment of signals related to the shape of the laser beam, G. Grehan and G. Gouesbet, Applied Optics 25 (1986) 3527-3538, have developed a system for measuring the particle size and velocity using a top-hat beam technique. Measurement of the diameter and velocity of particles can also be obtained from the phase shift of the scattered laser radiation as described for example by W. D. Bachalo (U.S. Pat. No. 4,854,705), P. Buchhave, J. Knuhtsen and P. E. S. Olidag (U.S. Pat. No. 4,701,051) and T. A. Hatton and J. L. Plawsky (U.S. Pat. No. 4,662,749).
These prior art techniques give unreliable diameter measurements when the particles are not spherical, which is common in thermal spray processes when particles are not fully molten. Other approaches use more than one laser beam at different wavelengths and, from the intensity and/or polarization of the scattered radiation, the diameter and velocity of the moving particles are determined (for example, J. C. Wang and K. R. Henken in Applied Optics 25 (1986) 653-657, and U.S. Pat. No. 4,854,705 by W. D. Bachalo).
Different techniques have been used to measure particle parameters in thermal spray processes, including the particle temperature. Simultaneous measurement of particle size, velocity and temperature has been carried out by J. R. Fincke, W. D. Swank, C. L. Jeffery and C. A. Mancuso in Meas. Sci. Technol., 4 (1993) 559-565 in jets of plasma-sprayed particles. Particle size and velocity are obtained from a combination laser sizing system and laser Doppler velocimeter while the particle temperature is determined by two-color pyrometry. S. M. Guselnikov, A. G. Zavarzin, V. P. Lyagushkin, M. Mikhalchenko and O. P. Solonenko in Plasma Jets, Solonenko and Fedorchenko (EDS), VSP, 1990, p. 163-170 used a combination of the two-focus anemometry for velocity measurement, laser forward scattering technique for size measurement and two-color pyrometry for temperature measurement. In both cases, the optical arrangement is relatively complex and hardly usable in industrial environment.
Another approach has been described by J. R. Fincke, C. L. Jeffery and S. B. Englert in J. Phys. E: Sci. Instrum., 21 (1988) 367-370 in which the temperature and diameter of sprayed particles are measured using a laser beam. The temperature is obtained using the two-color pyrometry while the diameter is computed from the intensity of the scattered beam after deconvolution to take into account of the gaussian shape of the laser beam.
Two systems have been proposed to measure the particle size, velocity and temperature based on the detection of thermal radiation emitted by the hot incandescent sprayed particles passing through a volume of measurement of known dimensions. In both cases, the temperature is evaluated by two-color pyrometry and the velocity is computed from the time of flight of the particles in the volume of measurement.
In the approach developed by T. Sakuta, T Ohtsuchi, K. Sakai and T. Takashima, Proc. Jpn. Symp. Plasma Chem. 4 (1991) 175-180, the diameter is obtained from the rise time of the detected signals when the particles enter and exit the volume of measurement.
In the approach developed by K. R. Hencken, D. A. Tichenor and J. C. F. Wang (U.S. Pat. No. 4,441,816), particles are seen through a double-slit mask. The first slit is narrow in such a way that only a fraction of the section of the moving particle is seen by the detectors. The second slit is larger than the particle image so that the entire section of the particle is seen. The velocity is obtained from the transit time of the particles in this second slit while the diameter is computed from the ratio of the radiation intensities collected in each slit. Since the first slit must be narrower than the image of the smallest particle to be analyzed, the transit time in this slit becomes very short when particles are moving at high velocity requiring very fast photodetectors and acquisition electronics components. In both approaches, a laser beam must be focused at the center of the volume of measurement to trigger the acquisition electronics only when a particle is traveling in the focal plane of the collection optics.
An object of the invention is to alleviate the aforementioned problems.